Eye on the future

We are pushing the limits of technology…

Eye on the future

One day, he is fixing radiators at home. The next day, he is trying to reduce the thermal noise in “the most sensitive scientific instrument ever constructed,” thousands of miles away in the US. When he gets to work at the University of Glasgow, Professor Giles Hammond is not just any other astrophysicist, but a DIY enthusiast, happy to repair the central heating and build garden walls, or “tinker” with some of the most critical components in the Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) detectors.

Even though “do-it-yourself” skills have played a key role in the history of the detectors, the Advanced LIGO project draws on the resources of an international team of over 1,000 leading researchers. The ultimate goal is a big one – the quest to detect gravitational waves – but Hammond has to focus on the tiniest details, at the same time as keeping his eye on the stars.

“We are developing a new field of astronomy,” Hammond explains. “Our eyes can see the stars and we’ve developed ways of seeing electromagnetic radiation (including optical, infrared, gamma ray, etc.), but laser interferometry is a whole new way of looking at the Universe – for example, understanding how a massive star becomes a black hole in the first place.” Einstein’s General Theory of Relativity is a very sophisticated theory, but all the smartest theories in the Universe need evidence to prove they are correct. Gravitational waves are one of the last unproven predictions of the theory, so if the detectors themselves don't work, or aren't sensitive enough to detect gravitational waves in the first place, you simply measure noise and not the signals.

To help the group of scientists at LIGO “see through space and time,” Hammond specialises in designing and installing a critical piece of equipment inside the detector – the mirror suspensions. In simple terms, as light reflects from mirror to mirror, inside the detector, the instruments measure the distance between the two mirrors, and if a gravitational wave hits the Earth, it moves the mirrors slightly, thus changing the distance between them. The problem, however, is that thousands of other events also have an effect on the mirrors – for example, an earthquake, a train or even just somebody dropping a spanner. The instruments themselves can create lots of noise, and the challenge is to recognise the signature of different types of noise (including the “jiggling” motion of the atoms that make up the mirrors) so they can be taken out of the equation, using clever mathematics and smart engineering.

The innovative solution was to hang the mirrors using fused silica glass fibres, instead of metal wire, and to suspend four mirrors in a quadruple pendulum – like a vertical necklace – to further dampen the vibrations. One of the biggest contributions of the Glasgow team is the silica fibres themselves, which are made in a special machine in the lab using a process very similar to teasing out wool. First used in the “monolithic stages” of the GEO600 detector in Germany, then later in Advanced LIGO, silica is used for a number of reasons, including the fact that it has a high melting point – 2,000 degrees Centigrade – which means it is one of the most stable forms of glass. Even though the fibres are extremely thin – 400 microns in diameter – they also have to be strong enough to hold up the mirrors, which each weigh 40kg. Part of the new design features new jointing techniques, getting the mirrors to bond to the fibres as if they are a single component, to minimise thermal noise. To illustrate the special properties of the silica fibres, Hammond also explains that thermal vibrations “pluck” the fibres, and it takes several days for the motion to “decay,” unlike the vibration on guitar strings which only lasts one or two seconds.

“We are pushing the limits of technology,” says Hammond. And the net result of meeting this “experimental challenge”, at frequencies around 30Hz, is an improvement of roughly 100 times compared to previous wire-based suspensions in the first LIGO, making it possible to eliminate almost every source of noise they can identify. Even the Earth tides, elastic deformations of the Earth due to the Moon and Sun, which stretch and shrink the Earth by around 400 millionths of a metre along the arms of Advanced LIGO, can be countered by moving the suspensions and seismic isolation systems inside the vacuum chambers. Hammond also stresses that the quest to eliminate noise will continue in future detectors, but says it's reassuring that they have already had a significant breakthrough with the upgraded version of LIGO, proving that the basic technology works.

According to Professor Martin Hendry, Head of the School of Physics and Astronomy at the University of Glasgow and the Chair of the Education and Public Outreach Group in LIGO, Hammond has made “significant contributions to the development of the monolithic stages of the Advanced LIGO quadruple pendulums, and also pioneered the development of the fused silica suspension for the prototype interferometer in Hanover, as well as new techniques to provide continued fused silica suspension support for future upgrades.”

As well as pioneering new technologies, Hammond also helps install the mirror suspensions in the detectors, after several rehearsals in Glasgow. “You can’t just turn up with a bunch of components then bolt them together and hope for the best,” says Hammond. “We have to make sure that we get it right every time, first time.” No matter how much they may practice, however, something can always go wrong – for example, minor accidents such as dropping a bolt on the suspensions during installation. “But these are complicated machines,” explains Hammond, “so this is just part of the learning process, and recovering from such incidents is important for developing robust engineering protocols.”

As soon as one problem is solved, Hammond and his colleagues are thinking ahead to the next technological challenge – keeping one eye on the future of the science as a whole. The Advanced LIGO detector has delivered a startling result by being the first to detect gravitational waves, but this is just a step along the way. For example, there are plans to build a new machine called the Cosmic Explorer, with 30-40km arms, compared with LIGO’s current 4km arms, to amplify the signal and improve detection. In Europe, the proposal is to build the Einstein Telescope, several hundred metres underground; while in Japan, the KAGRA detector will operate at around 20 degrees above absolute zero (-253° Centigrade), to eliminate one of the most significant sources of noise – thermal fluctuations. The LIGO infrastructure is 20 years old now, and even though it managed to accommodate the upgrade which led to the breakthrough in September 2015, it needs constant care and attention – an issue in the minds of those designing the next generation of super detectors.

Spin-offs

Hammond's research also includes the development of new techniques to “characterise the mechanical stress in fused silica suspension elements, grow silicon/sapphire crystalline fibres and analyse the performance gains for suspension upgrades to Advanced LIGO and future third-generation detectors.” He has also been closely involved in the development of low-cost, ultra-sensitive gravimeters, which utilise “soft springs” to provide the lowest resonant frequency MEMS (micro-electro-mechanical system) devices in the world, capable of measuring tiny fluctuations in gravity – for example, to monitor volcanic activity or search for hydrocarbons.

Just one of the spin-offs from gravitational-wave research, these tiny devices, called “Wee-g,” are just a few centimetres across (small enough to be mounted on drones) and will cost only a few hundred pounds, compared to about £80,000 for conventional systems. The new designs are based on MEMS technology, using a sophisticated version of the accelerometers in smartphones which tell up from down, to compensate for tidal fluctuations – i.e., the surface of the Earth goes up and down twice a day by about 40cm because of the pull of the Moon, and this affects the accuracy of subterranean measurements.

Developed in Glasgow by QuantIC, a “quantum technology hub,” which brings together 120 researchers and industry partners, Wee-g “opens up the possibility of making gravity measurement a much more realistic proposition for all kinds of industries,” says Hammond's colleague, Dr Richard Middlemiss. As well as being used for undersea exploration, the gravimeters will be used in nanosatellites, so they know exactly where they are whilst in orbit – a solution now being developed for Glasgow-based CubeSat developer Clyde Space. Schlumberger is another potential customer, using the gravimeters for oil exploration; while QinetiQ is interested in using the devices in defence aerospace and security systems.

Innovative architecture

Hammond first developed his interest in gravitational waves while doing his PhD at the University of Birmingham, before moving to Colorado, focusing on seismic isolation solutions. He then returned to Birmingham and later moved to Glasgow to develop the mirror suspensions. During this time, he has also been a member of the “thermal noise materials group,” focusing on thermal dynamics and the internal friction caused by thermal noise, which can interfere with the signals. Once you think you've got rid of the thermal noise, says Hammond, another problem is discovered – for example, thermal-elastic noise – and a further refinement is needed. That is what makes Hammond's work so challenging and motivates him and his colleagues.

When he was at school, Hammond wanted to become an architect, but good results in maths, physics and chemistry, combined with an early interest in astronomy, persuaded him to go in a different direction, and now he is the “architect” of very smart components which enable us to look through a new kind of window – and see the Universe as never before. His DIY experience has also been useful, teaching him how he should learn from mistakes – as well as push the boundaries. “Sometimes it’s very frustrating,” he says. But when the boundaries are billions of lights years away, it’s probably worth all the hassle.

Biography

Professor Giles Hammond is a Fellow of the Institute of Physics and an alumnus of the RSE Young Academy of Scotland. Together with his colleague, Dr Angus Bell, he led the implementation on site of the silica fibre suspension elements essential to the low-noise operation of the Advanced LIGO detectors. He has also led the application of gravitational-wave technology in other fields – in particular the development of small micro-electro-mechanical system (MEMS) devices that can be used as ultra-sensitive, portable gravimeters suitable for use in applications such as gravity imaging and seismology.